1. Field of the Invention
The present invention relates to a method and apparatus for non-contacting identification of the temperature distribution in a non-uniform examination subject which may use of phase and amplitude measurements of attenuated microwave radiation directed at the examination subject.
2. Description of the Prior Art
The temperature distribution of an examination subject is of particular interest in the medical field, wherein the examination subject is a human patient. It is possible to analyze the temperature distribution of a patient to identify areas exhibiting a pathological condition because diseased tissue exhibits a different heat dissipation from healthy tissue. It is also possible using a hyperthermia method to undertake local heating of an area affected by a nidus, for example, a tumor, in order to achieve decomposition of diseased cells, particularly in combination with radiation therapy. Under such treatment methods, the heating cannot exceed a critical limit value, and must be topically within the diseased area so that no healthy tissue is damaged. In a hyperthermia method, the temperature distribution is also of interest to assist in precisely localizing the area of heat application, and monitoring the success of the therapeutic measures.
As a consequence of the slight temperature differences which occur in the human body, methods for identifying the temperature distribution in humans must do so with a high precision. This requirement is difficult to meet, however, because, given a dielectrically non-uniform examination subject, such as the human body, the distribution of the complex dielectric constant in the examination subject enters significantly into the measured result, not only for measuring the characteristic radiation of the subject, but also measuring the actual temperature.
In the human body, for example, the dielectric constant varies between a value .epsilon.=10 for fat tissue up to a value .epsilon.=60 for bone. Because of the different dielectric constants , the characteristic radiation which emanates from a point in the examination subject is differently attenuated and refracted in the tissue, so that it is not possible to obtain an unambiguous result outside of the examination subject. Given the same temperature, a region within the examination subject having a high dielectric constant radiates more strongly than a region having a low dielectric constant. The intensity of the characteristic region outside of the examination subject is thus a function of the distribution of the dielectric constant and of the temperature. Making the assumption that the dielectric constant corresponds, for example, to an average value, causes incorrect results. For example, it is possible that a first area in the examination subject measured at 40.degree. C. on the basis of the characteristic radiation is in reality colder that a second area measured at 37.degree. C. If the first area measured at 40.degree. C. has an extremely high dielectric constant, its characteristic radiation in comparison to the area measured at 37.degree. C. will be incorrectly evaluated, the latter area having a low dielectric constant in accordance with the assumption.
A method and apparatus of the type described above are disclosed in the article "Aperture Synthesis Thermography:" Haslam et al, IEEE Transactions on Microwaves, Vol. MTT-32, No. 8, August 1984. The characteristic thermal radiation of an examination subject in this method and apparatus is received by an antenna. The received signal is divided in terms of amplitude and phase in a computer, these values serving for calculating the temperature distribution. The complex relationship with the dielectric constant and the problems resulting therefrom with respect to the reliability of the measured results are not discussed in detail in this publication.